Unable to cast object of type 'System.DBNull' to type 'System.String' occurs when you run the "Synchronize with CRM" function in the Microsoft Dynamics CRM 2011 Outlook Client

Use the following steps to resolve this issue on a Windows XP machine:

1. Close Microsoft Office Outlook
2. Browse to C:\Documents and Settings\<Username>\Application Data\Microsoft\MSCRM\Client and delete the contents of this folder, which will include the local SDF files
3. Browse to C:\Documents and Settings\<Username>\Local Settings\Application Data\Microsoft\MSCRM\Client and delete the contents of this folder, which will include the local SDF files
4. Start Microsoft Office Outlook
5. When you start Outlook for the first time after deleting the SDF files, you will notice some slow performance. This is expected because the SDF files are being recreated.

Use the following steps to resolve this issue on a Windows Vista or Windows 7 machine:

1. Close Microsoft Office Outlook
2. Browse to C:\Users\<Username>\AppData\Roaming\Microsoft\MSCRM\Client and delete the contents of this folder, which will include the local SDF files
3. Browse to C:\Users\<Username>\AppData\Local\Microsoft\MSCRM\Client and delete the contents of this folder, which will include the local SDF files
4. Start Microsoft Office Outlook
5. When you start Outlook for the first time after deleting the SDF files, you will notice some slow performance. This is expected because the SDF files are being recreated.